The use of NMR techniques in measurement, detection and imaging has become desirable in many scientific fields of endeavor. The non-invasive, non-destructive nature of NMR has facilitated application to industrial instrumentation, analysis and control tasks, in a variety of applications, including but not limited to cosmetics, perfumes, industrial chemicals, biological samples and food products. As one example, check weighing is used by the pharmaceuticals industry for monitoring and regulating the amount of drug in a sealed glass vial during filling. The drug weight can be as small as a fraction of a gram, and is required to be weighed with an accuracy of a few percent or better, in a vial weighing tens of grams at a rate of several weighing per second.
International Patent Application No. WO99/67606, incorporated herein by reference as if fully written out below, describes a check weighing system for samples on a production line using NMR techniques. This system includes a magnet for creating a static magnetic field over an interrogation zone to produce a net magnetisation within a sample located within the interrogation zone, and a RF coil for applying an alternating magnetic field over the interrogation zone to cause excitation of the sample according to the principles of NMR.
As is well known in the general NMR art, successful application of magnetic resonance requires that the RF coil generate a uniform magnetic field at the location of the sample protons under test, and that the RF coil have minimal sensitivity to electrical interference (i.e., noise) from external sources. A variety of basic RF coil configurations have been utilized in NMR systems to satisfy these constraints, including helical, saddle and Helmholz. However, in continuous production line applications additional constraints make such shapes unusable.
For example, in continuous production line applications where a plurality of closely proximate packages move continuously, the field produced by the RF coil at the site of packages neighboring the sample under test must be minimized to avoid influencing the field and measurement of the sample under test (i.e., reduce cross coupling). This is a significant concern in applications such as the packaging of pharmaceuticals where rows of drug vials move simultaneously down the production line.
Pharmaceutical packaging is also exemplary of another significant constraint upon RF probes. Clean environmental restrictions require the unimpeded airflow in the vicinity of the vials, and any RF probe utilized in this application must be designed for minimal airflow obstruction.
It is desirable to provide a RF probe apparatus for an NMR check weighing system that produces a uniform magnetic field at the test location (preferably the center of the RF probe), has minimal sensitivity to electrical interference from external sources, shapes the magnetic field to minimize cross coupling from packages not under test, and presents minimal airflow obstruction.